deps: bump min version of google-api-core and google-cloud-core to 2.x

[tswast]

Fixes constraints file to match setup.py

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Towards googleapis/python-aiplatform#4079 🦕

[Linchin]

Thank you so much for fixing the issue so quickly!

For posterity, what is the process to update the required dependency versions?

[tswast]

For posterity, what is the process to update the required dependency versions?

@Linchin Main thing is that when updating setup.py, make sure to update constraints.txt for the minimum supported Python version. Note: the tests validate via constraints.txt, so if you find yourself wanting to update one of those, make sure you make the corresponding change in setup.py.

See: googleapis/gapic-generator-python#1788 for a hint at how this works in the generated clients as well as googleapis/synthtool#1901 to apply these minimum version constraints checks via the synthtool templated files.

[tswast]

I've filed #1974 for another layer of automated testing.